Growing up in Upstate NY, I always assumed part of it was also due to the fact that breaks during the winter basically translate to hours spent cooped-up inside, watching TV or playing on the computer.
I know that the 3-month summer break stems from historical harvest/planting schedules... but when there's 20 inches of snow outside and the wind chill is -10 F, I'll take 3 months of summer vacay over 6 weeks of summer and 6 weeks of winter breaks any day.